Solar System’s Heliosphere Much More Compact, Rounded than Previously Thought

Many stars show tails that trail behind them like a comet’s tail, supporting the idea that our own Solar System has one too. However, new evidence from NASA’s Cassini, Voyager and Interstellar Boundary Explorer (IBEX) missions suggests that the trailing end of the Solar System may not be stretched out in a long tail. The research is published in the journal Nature Astronomy.

A compact model of the heliosphere. Image credit: Dialynas et al.

The Sun releases a constant outflow of magnetic solar material (solar wind) that fills the inner Solar System, reaching far past the orbit of Neptune. This solar wind creates a bubble, some 23 billion miles across, called the heliosphere.

The prevalent picture of the heliosphere was one of comet-shaped structure, with a rounded head and an extended tail.

But new data covering an entire 11-year solar activity cycle show that may not be the case: the heliosphere may be rounded on both ends, making its shape almost spherical.

“Instead of a prolonged, comet-like tail, this rough bubble-shape of the heliosphere is due to the strong interstellar magnetic field — much stronger than what was anticipated in the past — combined with the fact that the ratio between particle pressure and magnetic pressure inside the heliosheath is high,” said lead author Dr. Kostas Dialynas, a researcher at the Academy of Athens in Greece.

Cassini’s Ion and Neutral Camera (INCA) has given the researchers crucial new clues about the shape of the heliosphere’s trailing end, often called the heliotail.

When charged particles from the inner Solar System reach the boundary of the heliosphere, they sometimes undergo a series of charge exchanges with neutral gas atoms from the interstellar medium, dropping and regaining electrons as they travel through this vast boundary region.

Some of these particles are pinged back in toward the inner Solar System as fast-moving neutral atoms, which can be measured by INCA.

“INCA was designed to image the ions that are trapped in the magnetosphere of Saturn,” said co-author Dr. Tom Krimigis, an instrument lead on NASA’s Voyager and Cassini missions based at Johns Hopkins University’s Applied Physics Laboratory.

“We never thought that we would see what we’re seeing and be able to image the boundaries of the heliosphere.”

Because these particles move at a small fraction of the speed of light, their journeys from the Sun to the edge of the heliosphere and back again take years.

So when the number of particles coming from the Sun changes — usually as a result of its 11-year activity cycle — it takes years before that’s reflected in the amount of neutral atoms shooting back into the Solar System.

Cassini’s new measurements of these neutral atoms revealed something unexpected — the particles coming from the tail of the heliosphere reflect the changes in the solar cycle almost exactly as fast as those coming from the nose of the heliosphere.

“If the heliosphere’s ‘tail’ is stretched out like a comet, we’d expect that the patterns of the solar cycle would show up much later in the measured neutral atoms,” Dr. Krimigis noted.

But because patterns from solar activity show just as quickly in tail particles as those from the nose, that implies the tail is about the same distance from us as the nose.

This means that long, comet-like tail that scientists envisioned may not exist at all — instead, the heliosphere may be nearly round and symmetrical.

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K. Dialynas et al. 2017. The bubble-like shape of the heliosphere observed by Voyager and Cassini. Nature Astronomy 1: 0115; doi: 10.1038/s41550-017-0115

This article is based on a press-release from the National Aeronautics and Space Administration.